1. Field of the Invention
The present invention is directed towards a method and apparatus for producing better molds for making precisely-contoured thermoplastic products of ectremely consistent surface profile and smoothness, in general. In particular, optical plastic products, such as vision-corrective lenses (implant interocular, contact lens, prescription spectacle lens, reading glasses, magnifier lens), and information storage optical disks (digital audio compact disks, video disks, interactive video disks, CD/ROM, and write-once or erasble computer memory storage disks), as well as any number of photographic and instrument lenses requiring precise magnification or demagnification are specifically included in the field of the present invention.
2. Description of the Related Technology
Optical plastic products are typically formed in one or more of the following ways:
(1) Low-pressure molding or casting processes in which typically a low-viscosity prepolymer or monomer mixture is polymerized while contained within a mold cavity;
(2) High-pressure molding or casting processes wherein the incoming plastic polymer is already of suitably high molecular weight and essentially complete structure and is heated to sufficient temperature to provide a flowable, viscous mass which can be formed into its ultimate configuration while being held under high pressures within a mold cavity, and cooled to an appropriately low temperature for solidification therein;
(3) Direct machining processes from a preformed shape or plastic stock material.
For either low-pressure casting or high-pressure molding processes, some of the requirements for the mold elements which form the optical surfaces of the final plastic product are the same; such part-forming optical mold elements must, of course, have precisely the desired contour or profile (including a shrinkage compensation factor therein) and an extremely smooth, highly polished microsurface (free of flaws and discontinuities). In addition to these optical quality requirements of the part-forming mold surfaces, which are requirements common to both low-pressure and high-pressure molding processes, there are additional special requirements for high-pressure thermoplastic molding:
(1) Much greater need for rigidity and dimensional stability because of the high pressures exerted within the mold cavity which quite commonly reach 5-10,000 PSI, and fast injection fill times which may take only 0.1-0.2 seconds, thus creating a shockwave in the mold cavity.
(2) Desirably high rates of heat transfer because inherent to high-pressure thermoplastic molding processes is the need in each molding cycle to remove large amounts of heat from the plastic as it goes from a temperature well above its melting point to a dimensionally stable temperature below its solidification point or glass-transition temperature point. Therefore, the part-forming mold elements commonly contain coolant-flow passages wherein suitable heat-transfer fluid is continuously circulated to assist in this function of rapid cooling.
(3) Desirably impervious mold surface resisting corrosion and scratching.
In addition, repairability and service life are considerations in the selection requirements for suitable part-forming optical mold elements.
Thus, some materials of construction and types of construction well suited for low-pressure molding and casting optical processes are unsuitable for high-pressure optical molding processes. For example, glass and certain ceramics are widely used in optical die construction for cast plastic lenses and sheets formed from crosslinking reactions (polymerizing the plastic while contained in such mold cavities via heat-initiated or radiation-initiated polymerization reactions). Glass and related ceramics are readily fabricated into almost any optical shape and surface of interest by very well known and established, relatively low-cost means. However, these glassy materials are not useful in high-pressure optical molding processes because they are heat insulators and relatively soft-surfaced, easily damaged materials of unsuitable mechanical properties.
Another class of optical mold materials widely used for low-pressure molding and casting processes include the electro-formed metals (usually nickel), which is produced from well-known die-replication processes off of a mirror image master or model piece. The heat-transfer rate of such electro-formed nickel mold surfaces is, of course, quite superior to glass, but these electro-formed mold elements are economically limited to relatively thin sections (30-150 mils) and, thus, are unsuited for high-pressure molding processes unless somehow assembled into a composite with a high-strength metal-backing material which usually also contains the coolant-flow channels. Combining such an optically acceptable part-forming surface element with such a reinforcing and cooling backing element is far from being a trivial challenge. First is the need to precisely mate the adjoining surfaces, to prevent stress concentrations and to provide a good interface for optimum heat transfer across that boundary. Also, since the nickel surface is of only moderate hardness and, thus, susceptible to scratches or other surface damage rendering the mold optically unsuitable without repair, some sort of provision for removing this surface element at a later time is required. The easier the replacement of such surface elements becomes, however, the more often compromised is the stability of the resulting composite assembly or its desirable heat-transfer rates.
For these reasons, high-pressure optical thermoplastic molding processes have, most commonly, employed monolithic metal (varying grades of tool steels, in particular) for materials of construction for the part-forming mold elements. These mold elements are fabricated in all the usual ways well known to the metalworking industry, including metal removal via milling, lathe turning, fly cutting, or spark erosion by electrical discharge. Once the nominal dimensions, shape or contour of the fabricated steel mold element have been attained, then the part-forming surfaces are abrasively lapped by successively finer abrasives in a manner well known to those skilled in the art until these contoured surfaces reach satisfactory degrees of smoothness and polish.
An anomaly in this mold fabrication process is that in the first stages of machining processes directed towards achieving nominal dimensions and contour, a relatively-soft, easily-worked material is desired (or required in some cases, such as single-point diamond turning--most tool steels cannot be fabricated in such a way, this fabricating technique thus limiting itself to relatively-softer materials). Yet, abrasive lapping and polishing operations favor relatively-harder materials. In particular, softer surfaces tend to develop an "orange peel" appearance and macro texture undesirable for optical surfaces.
The optical mold surfaces which form the part should desirably be of a material which is very hard (to resist scratching), chemically inert in its ordinary environment (to prevent rusting, oxidation or tarnish which renders the surface optically unacceptable), and of suitable metallurgical purity (of a highly regular and dense-grain structure-free of slag, impurities, voids, or other optically unacceptable microflaws).
So far, stainless steel of suitably-refined metallurgical purity grades has become the material of choice. Its resistance to mechanical deletion and load-bearing strength is very high, and generally its mechanical properties represent a useful balance between reasonably fast metal-removal machining rates and reasonable ease in lapping to a wrinkle-free optical surface polish. Its chemical inertness is well-known. Furthermore, at premium costs, stainless steel grates of high metallurgical purity can be obtained via ESR (electro-slag refining) or VIM-VAR (vacuum induction melting-vacuum arc remelt) processes. Its thermal conductivity is high when compared to glass or electroformed nickel composite mold elements, and these monolithic stainless steel mold elements can be readily fabricated (including coolant flow channels) via well-known techniques of machining and lapping.
However, certain deficiencies remain. Stainless steel is not the hardest nor most chemically inert of metal surfaces. Although it can be metallurgically purified to a great extent, it is still subject to random impurities and voids of an optically objectionable nature. Its heat transfer rates, while acceptable, are vastly lower than certain other soft metals such as copper, aluminum, or silver and gold. Also, successive cycles of surface damage and repair (via regrinding and repolishing) reduce the mold element nominal dimensions until eventually the resulting product is out of tolerance (such as by means of increased molded plastic part thickness because of successive mold resurfacing operations). For example, digital audio compact disk molded-part thickness tolerances are only + or - 0.004 inches.
There have been two previous attempts to combine, in a two-part assembly, metal materials of dissimilarly high and low thermal conductivities, to produce optical lens injection molds for thermoplastics. However, in both references the stated intention of this assembly is to compensate for the slower cooling rates (inherent in powered lenses) of the thicker parts of inherently nonuniform cross-section. Thus the associated construction of these assemblies shows that the junction of the dissimilar metal components follow their developer's concern for nonuniform heat distribution caused by the nonuniform lens cross-sectional thickness, rather than any concern for an unacceptably slow overall cooling rate.
Laliberte, in U.S. Pat. No. 4,364,878 (column 5, lines 9-30), discusses a two-piece mold insert assembly in passing.
Williams, in U.S. Pat. No. 2,292,917, discloses a two-piece mold insert assembly at much more length, first establishing that ". . . highly polished dies are usually formed of steel. . . " (page 1, lines 14-15), which are apparently his material of choice for mold dies 1 and 2, which actually are in contact with the molten plastic lens, and these dies in turn are joined (by mating surface contours) and backed by bodies 19 and 20 which are . . . "formed of a material, such as copper, of thermal conductivity considerably higher than that of the material of the dies" (page 3, lines 71-74).